Process for production of a carboxylic acid/diol mixture suitable for use in polyester production

ABSTRACT

In this invention, a process is provided by which a carboxylic acid/diol mixture suitable as starting material for a polyester production is obtained from a decolorized carboxylic acid solution without isolation of a substantially dry carboxylic acid solid. More specifically, in this invention, a process is provided by which a terephthalic acid/ethylene glycol mixture suitable as starting material for a polyester production is obtained from a decolorized terephthalic acid solution without isolation of a substantially dry terephthalic acid solid.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. applicationSer. No. 10/271,058, filed Oct. 15, 2002, hereby incorporated byreference in its entirety to the extent that they do not contradictstatements herein.

FIELD OF INVENTION

The present invention relates to a process by which a carboxylicacid/diol mixture is obtained from a decolorized carboxylic acidsolution without isolation of a substantially dry carboxylic acid solid.More specifically, the present invention relates to a process by which aterephthalic acid/ethylene glycol mixture suitable as a startingmaterial for polyester production is obtained from a decolorizedterephthalic acid solution without isolation of a substantially dryterephthalic acid solid.

BACKGROUND OF THE INVENTION

Thermoplastic polyesters are step growth polymers that are useful whenmade to high molecular weights. The first step in a common method ofproducing a polyester such as polyethylene terephthalate (PET) is anesterification or ester-exchange stage where a diacid (typicallyterephthalic acid) reacts with an appropriate diol (typically ethyleneglycol) to give a bis(hydroxyalkyl)ester and some linear oligomers.Water is evolved at this stage and is usually removed by fractionaldistillation.

Pursuant to the goal of making polyethylene terephthalate and otherpolyesters, a great deal of patent literature is dedicated to thedescribing processes for preparing terephthalic acid/ethylene glycolmixtures suitable as starting material. In general, these inventionsdescribe specific mixing schemes with a purified terephthalic acid solidand liquid ethylene glycol as starting materials. Additionally, there issubstantial body of literature devoted to producing a purifiedterephthalic acid in the powder form that is suitable for use inproducing PET. The objective of this invention is to describe a processby which a terephthalic acid/ethylene glycol mixture suitable asstarting material for polyester production is obtained from adecolorized terephthalic acid solution without isolation of asubstantially dry terephthalic acid solid.

A number of processes for producing the purified terephthalic acid solidhave been developed and are commercially available. Usually, thepurified terephthalic acid solid is produced in a multi-step processwherein a crude terephthalic acid is produced. The crude terephthalicacid does not have sufficient quality for direct use as startingmaterial in commercial PET. Instead, the crude terephthalic acid isusually refined to purified terephthalic acid solid.

Liquid phase oxidation of p-xylene produces crude terephthalic acid. Thecrude terephthalic acid is dissolved in water and hydrogenated for thepurpose of converting 4-carboxybenzaldehyde to p-toluic acid, which is amore water-soluble derivative, and for the purpose of convertingcharacteristically yellow compounds to colorless derivatives. Any4-carboxybenzaldehyde and p-toluic acid in the final purifiedterephthalic acid product is particularly detrimental to polymerizationprocesses as they act as a chain terminator during the condensationreaction between terephthalic acid and ethylene glycol in the productionof PET. Typical purified terephthalic acid contains on a weight basisless than 25 parts per million (ppm) 4-carboxybenzaldehyde and less than150 ppm p-toluic acid.

The crude terephthalic acid typically contains on a weight basis fromabout 800 to 7,000 parts per million (ppm) 4-carboxybenzaldehyde andabout 200 to 1,500 ppm p-toluic acid as the main impurities. The crudeterephthalic acid also contains lesser amounts, about 20-200 ppm range,of yellow color aromatic compounds having the structures of benzil,fluorenone, and/or anthraquinone, which are characteristically yellowcompounds as impurities resulting from coupling side reactions occurringduring oxidation of p-xylene. It is necessary to purify the crudeterephthalic acid when using it as a starting material for producingpolyester fiber, which requires a purified terephthalic acid as astarting material.

Such a purification process typically comprises adding water to thecrude terephthalic acid to form a crude terephthalic acid solution,which is heated to dissolve the crude terephthalic acid. The crudeterephthalic acid solution is then passed to a reactor zone in which thesolution is contacted with hydrogen in the presence of a heterogeneouscatalyst at temperatures of about 200° to about 375° C. This reductionstep converts the various color bodies present in the crude terephthalicacid to colorless products. The principal impurity,4-carboxybenzaldehyde, is converted to p-toluic acid.

Typical crude terephthalic acid contains excessive amounts of both4-carboxybenzaldehyde and p-toluic acid on a weight basis. Therefore, toachieve less than 25 ppmw 4-carboxybenzaldehyde and less than 150 ppmwp-toluic acid in the purified terephthalic acid requires mechanisms forpurifying the crude terephthalic acid and removing the contaminants.

Subsequent separation and isolation of the purified terephthalic acidcan be accomplished via a wide variety of separation methods includingcrystallization, centrifugation, filtration, extraction and combinationsthereof followed by drying. These processes are described in U.S. Pat.Nos. 4,500,732; 5,175,355; and 5,583,254; all of which are hereinincorporated by reference. It is necessary to perform a separation stepdue the nature of the crude terephthalic acid feedstock to thehydrogenation process.

A number of processes have been developed for producing a purifiedterephthalic acid solid from crude terephthalic acid. In general, thecommon features among these processes are as follows:

Step (1) is decolorization of the crude terephthalic acid usually viahydrogenation treatment in an aqueous medium;

Step (2) is purification/separation of the terephthalic acid frompartial oxidation products usually via fractional crystallizationfollowed by liquor exchange with contaminant-free water; and

Step (3) is production of a solid purified terephthalic acid productwith consistent material handling properties usually via crystallizationof terephthalic acid followed by drying of purified terephthalic acidfrom water.

The resultant purified terephthalic acid powder along with ethyleneglycol are starting materials in the production of polyestersspecifically PET. Because the difficulty in handling, mixing, anddissolving terephthalic acid solids, the purified terephthalic acidsolid is usually mixed with ethylene glycol to form a paste prior tointroduction into an esterification reactor system.

In the present invention, a novel process has been discovered resultingin fewer steps than the currently employed processes. The primaryutility of the invention is reduction of capital and operating costsassociated with the isolation of a terephthalic acid powder. In theconventional approach toward producing terephthalic acid, thepost-hydrogenated aqueous solution is passed to a series of crystallizervessels for the purpose of purifying the terephthalic acid bycrystallization and for the purpose of obtaining a uniform particle sizedistribution necessary for good flowability of purified terephthalicpowder. Further, the p-toluic acid contaminated mother liquor from thecrystallization process must be removed prior to a drying step toisolate the purified terephthalic powder.

In on embodiment of the present invention, the crude terephthalic acidsolution with low concentrations of p-toluic acid and4-carboxybenzaldehyde is hydrogenated to form a decolorized terephthalicacid solution. Starting with crude terephthalic acid with lowconcentrations of the p-toluic acid and 4-carboxybenzaldehyde eliminatesthe need for separation of p-toluic acid-contaminated mother liquor fromthe terephthalic acid. Hence, the decolorized terephthalic acid solutioncan be directly combined with ethylene glycol in an esterification zoneto produce a terephthalic acid/ethylene glycol mixture. By bypassingconventional processes for producing a purified terephthalic acidpowder, the need for the equipment necessary to purify and isolatepurified terephthalic powder is eliminated.

Another surprising and seemingly contradictory aspect of the inventionis the benefits of addition of large amounts of water to theesterification reaction starting materials. This is directly contrary toaccepted esterification procedures. The esterification reaction:RCOOH+R′OH→RCOOR′+H₂Ois generally not complete. The water formed in the course of thereaction tends to react with the ester to hydrolyze it, i.e. toregenerate the original alcohol and acid. In order to drive the reactiontoward the ester, the prior art teaches removal of water from the systemby a variety of methods such as distillation or dehydration with ahydrophilic compound. According to conventional esterificationprocedures, it is non-intuitive to add large amount of the water to theacid/alcohol starting material.

SUMMARY OF THE INVENTION

The present invention relates to a process by which a carboxylicacid/diol mixture is obtained from a decolorized carboxylic acidsolution without isolation of a substantially dry carboxylic acid solid.More specifically, the present invention relates to a process for theproduction of a terephthalic acid/ethylene glycol mixture suitable asfeedstock for the production of commercial PET. The resulting processhas fewer steps than currently employed processes and can be operated atlower operating cost and constructed at lower capital cost.Specifically, the present invention incorporates a direct displacementof water with ethylene glycol step following hydrogenation treatment ofcrude terephthalic acid. Incorporation of the displacement stepeliminates the need to isolate a purified terephthalic acid solidthereby eliminating the need for crystallization, solid-liquidseparation, and solids handling equipment normally found in commercialpurified terephthalic acid processes.

It is an object of this invention to provide a process for producing acarboxylic acid/diol mixture without isolation of a substantially drycarboxylic acid solid.

It is another object of this invention to provide a process forproducing a terephthalic acid/diol mixture without isolation of asubstantially dry terephthalic acid solid.

It is another object of this invention to provide a process forproducing a terephthalic acid/ethylene glycol mixture without isolationof a substantially dry terephthalic acid solid.

It is another object of this invention to provide a process forproducing a terephthalic acid/ethylene glycol mixture without isolationof a substantially dry terephthalic acid solid by vaporization of thewater from a decolorized terephthalic acid solution with enthalpysupplied by ethylene glycol in a esterification reactor.

It is another object of this invention to provide a process forproducing a terephthalic acid/ethylene glycol mixture without isolationof a substantially dry terephthalic acid solid by removing water from adecolorized terephthalic acid solution through the use of solid liquiddisplacement devices such as centrifuges, filters or cyclones.

In a first embodiment of this invention, a process for producing acarboxylic acid/diol mixture is provided, the process comprising addinga diol to a decolorized carboxylic acid solution in an esterificationreactor zone to remove a portion of the water to form the carboxylicacid/diol mixture; wherein said carboxylic acid and diol subsequentlyreacts in the esterification zone to form a hydroxy alkyl ester stream.Typically, the carboxylic acid is selected from a group consisting ofterephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, andmixtures thereof

In another embodiment of this invention, a process for producing acarboxylic acid/diol mixture is provided, the process comprising thefollowing steps:

(a) mixing a crude carboxylic acid powder with water in a mixing zone toform a crude carboxylic acid solution; wherein the carboxylic acid isselected from a group consisting of terephthalic acid, isophthalic acid,naphthalene dicarboxylic acid, and mixtures thereof;

(b) decolorizing the crude carboxylic acid solution in a reactor zone toproduce a decolorized carboxylic acid solution.

(c) optionally, flashing the decolorized carboxylic acid solution in aflashing zone to remove a portion of the contaminated water from thedecolorized carboxylic acid solution; and

(d) adding a diol to the decolorized carboxylic acid solution in anesterification reactor zone to vaporize a portion of the water to formthe carboxylic acid/diol mixture; wherein the carboxylic acid and diolsubsequently reacts in the esterification zone to form a hydroxy alkylester stream.

In another embodiment of this invention, a process for producing aterephthalic acid/diol mixture is provided, the process comprisingvaporizing a decolorized terephthalic acid solution with a diol in anesterification reactor zone to remove a portion of the water to form theterephthalic acid/diol mixture; wherein the terephthalic acid and diolsubsequently reacts in the esterification zone to form a hydroxyalkyester stream.

In another embodiment of this invention, a process for producing aterephthalic acid/diol mixture is provided, the process comprising thefollowing steps:

(a) mixing a crude terephthalic acid powder with water in a mixing zoneto form a crude terephthalic acid solution;

(b) decolorizing the crude terephthalic acid solution in a reactor zoneto form a decolorized terephthalic acid solution;

(c) optionally, flashing the decolorized terephthalic acid solution in aflashing zone to remove a portion of water from the aqueous terephthalicsolution; and

(d) adding a diol to the decolorized terephthalic acid solution in anesterification reactor zone to remove a portion of water to form theterephthalic acid/diol mixture; wherein the terephthalic acid and diolsubsequently reacts to from a hydroxy alkyl ester stream.

In another embodiment of this invention, a process for producing acarboxylic acid/diol mixture is provided, the process comprisingremoving a portion of the p-toluic contaminated water in an terephthalicacid aqueous slurry by adding a diol in a liquor removal zone to producesaid carboxylic acid/ethylene glycol mixture.

In another embodiment of this invention, a process for producing acarboxylic acid/diol mixture is provided, the process comprising thefollowing steps:

(a) mixing a crude carboxylic acid powder with water in a mixing zone toform a crude carboxylic acid solution;

(b) decolorizing the crude carboxylic acid in a reactor zone to producea decolorized carboxylic acid solution.

(c) crystallizing the decolorized carboxylic acid solution in acrystallization zone to form a terephthalic acid aqueous slurry; and

(d) removing a portion of the contaminated water in said terephthalicacid aqueous slurry by adding a diol in a liquor removal zone to producesaid carboxylic acid/diol mixture.

In another embodiment of this invention, a process for producing aterephthalic acid/ethylene glycol mixture is provided, the processcomprising removing a portion of the p-toluic contaminated water in anterephthalic acid aqueous slurry by adding a diol in a liquor removalzone to produce the terephthalic acid/diol mixture.

In another embodiment of this invention, a process for producing aterephthalic acid/ethylene glycol mixture is provided, the processcomprising the following steps:

(a) mixing a crude terephthalic acid powder with water in a mixing zoneto form a crude terephthalic acid solution;

(b) decolorizing the crude terephthalic acid solution in a reactor zoneto form a decolorized terephthalic acid solution;

(c) crystallizing of the decolorized terephthalic acid solution in acrystallization zone to form an terephthalic acid aqueous slurry; and

(d) removing a portion of p-toluic acid contaminated water in theterephthalic acid aqueous slurry by adding a diol in a liquor removalzone to produce the terephthalic acid/diol mixture.

These objects, and other objects, will become more apparent to otherswith ordinary skill in the art after reading this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of this invention. A process isprovided utilizing carboxylic acid powder to produce a carboxylicacid/diol mixture with the carboxylic acid and diol subsequentlyreacting to form a hydroxy alkyl ester stream

FIG. 2 illustrates an alternative embodiment of this invention. Aprocess is provided utilizing a terephthalic acid powder to produce aterephthalic acid/diol mixture with the terephthalic acid and diolsubsequently react to form a hydroxy alkyl ester stream

FIG. 3 illustrates another alternative embodiment of this invention. Aprocess is provided which utilizes a carboxylic acid powder to produce acarboxylic acid/diol mixture.

FIG. 4 illustrates yet another alternative embodiment of this invention.A process is provided where a crude terephthalic acid powder is utilizedto produce a terephthalic acid/diol mixture.

DETAILED DESCRIPTION OF THE INVENTION

An objective of this invention is to describe a process by which aterephthalic acid/ethylene glycol mixture suitable as starting materialfor polyester production is obtained from a decolorized terephthalicacid solution without isolation of a substantially dry terephthalic acidsolid.

In one embodiment of the invention substantially dry generally meansless than 5% by weight moisture. In another embodiment of the invention,another range can be less than 10% by weight moisture. In anotherembodiment of the invention, still another range can be less than 15% byweight moisture. In another embodiment of the invention, yet anotherrange for substantially dry can mean less than 20% by weight moisture.In another embodiment of the invention, another range for substantiallydry can mean less than 25% by weight moisture. In yet another embodimentof the substantially dry would be the moisture content obtained throughthe use of mostly mechanical means for drying and wherein the majorityof the drying is not accomplished through evaporation. Majority as usedherein means greater than 50%.

In the first embodiment of this invention a process for producing acarboxylic acid/diol mixture the process comprising the adding a diol toa decolorized carboxylic acid solution in an esterification reactor zoneto remove a portion of the water to form the carboxylic acid/diolmixture; wherein the carboxylic acid is selected from a group consistingof terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid,and mixtures thereof; wherein said carboxylic acid and diol subsequentlyreacts in the esterification zone to from a hydroxy alkyl ester stream.

The esterification reactor zone, the decolorized carboxylic acidsolution and a process to produce the decolorized carboxylic acidsolution is described subsequently in a second embodiment of thisinvention.

In the second embodiment of this invention a process for producing acarboxylic acid/diol mixture is provided as shown in FIG. #1.

Step (1) comprises mixing a crude carboxylic acid powder in conduit 105with water in conduit 115 in a mixing zone 110 to form a crudecarboxylic acid solution in conduit 120; Typically, the carboxylic acidis selected from a group consisting of terephthalic acid, isophthalicacid, naphthalene dicarboxylic acid, and mixtures thereof. The mixing ofthe crude carboxylic powder in conduit 105 with water in conduit 115 inthe mixing zone 110 can be accomplished by any means known in the art.The mixing zone 110 can be any vessel or equipment capable of mixing thecrude carboxylic acid powder. The temperature and pressure of the mixingzone 110 is that which is sufficient to properly slurry the crudecarboxylic acid powder in conduit 105 with water in conduit 115.Typically, the crude carboxylic acid powder in conduit 105 is slurriedwith water in conduit 115 in mixing zone 110 at a concentration of15-35% by weight.

Step (2) is decolorizing the crude carboxylic acid solution in conduit120 in a reactor zone 125 to produce a decolorized carboxylic acidsolution 135.

The decolorizing of the crude carboxylic acid solution in conduit 120can be accomplished by any means known in the art. Preferably, thedecolorizing can be accomplished by reacting the crude carboxylic acidsolution in conduit 120 with hydrogen in conduit 130 in the presence ofa. catalyst in a reactor zone 125 to produce a decolorized carboxylicsolution.

For the reactor zone 125, there are no special limitations in the formor construction thereof, subject to an arrangement that allows supply ofhydrogen in conduit 130 to effect intimate contact of the crudecarboxylic acid solution in conduit 120 with the catalyst in the reactorzone 125. Typically, the catalyst is usually a single Group VIII metalor combination of Group VIII metals. Preferably, the catalyst isselected from a group consisting of palladium, ruthenium, rhodium andcombination thereof. Most preferably, the catalyst is palladium.Typically, the catalyst is supported, preferably on porous carbon.

The reactor zone 125 comprises a hydrogenation reactor that operates ata temperature and pressure sufficient to hydrogenate thecharacteristically yellow compounds in the crude carboxylic acidsolution in conduit 120. By hydrogenation treatment, thecharacteristically yellow compounds in the crude carboxylic acidsolution are converted to colorless derivatives. The b* color of in thedecolorized carboxylic acid solution in conduit 135 is between about 0.5to about 4. Preferably the b* color of the carboxylic acid solution inconduit 135 is between 0.5 to 2.0. Most preferably the b* color in thecarboxylic solution in conduit 135 is between 0.5 to 1.5. The b* is oneof the three-color attributes measured on a spectroscopicreflectance-based instrument. The color can be measure by any deviceknown in the art. A Hunter Ultrascan XE instrument is typically themeasuring device. Positive readings signify the degree of yellow (orabsorbance of blue), while negative readings signify the degree of blue(or absorbance of yellow).

The hydrogen in conduit 130 is fed at a rate sufficient to convert thecharacteristically yellow compounds in the crude carboxylic slurry inconduit 120 to colorless derivatives; wherein the b* color is betweenabout 0.5 to about 4.0 in the decolorized carboxylic acid solution inconduit 135

Step (3) comprises, optionally, flashing the decolorized carboxylic acidsolution 135 in a flashing zone 145 to remove a portion of the waterfrom the decolorized carboxylic acid solution in conduit 135. Theflashing of the aqueous carboxylic solution 135 can be accomplished byany means know in the art. Typically, a vessel or a plurality of vesselsare used to accomplish the flashing. In the flashing zone 145, water andresidual hydrogen can be removed as a vapor via conduit 150. The flashvessel(s) operate at a temperature sufficient to remove a portion of thewater. Alternatively, flashing zone 145 can be omitted as indicated byconduit 140.

Step (4) comprises, adding a diol in conduit 170 to the decolorizedcarboxylic acid solution in conduit 155. A portion of the water viaconduit 165 is removed from an esterification reactor zone 160 to formsaid carboxylic acid/diol mixture in the esterification reactor zone160. In one embodiment of the invention, a portion can mean any part upto and including the whole. In another embodiment of the invention aportion can mean at least 5% by weight of the water is removed. Inanother embodiment of the invention a portion can mean at least 10% byweight of the water is removed. In another embodiment of the invention aportion can mean at least 25% by weight of the water is removed. Inanother embodiment of the invention a portion can mean at least 50% byweight of the water is removed. In another embodiment of the invention,a portion can mean at least 75% by weight of the water is removed. Inanother embodiment of the invention a portion can mean at least 85% byweight of the water is removed. In another embodiment of the invention aportion can mean at least 90% by weight of the water is removed. Thecarboxylic acid and diol subsequently reacts to form a hydroxy alkylester stream 175. The hydroxy alkyl ester stream 175 comprises a hydroxyalkyl ester compound.

The diol in conduit 170 is introduced in such a manner as to displacethe water as the dominant slurrying liquid. This can be accomplished byintroducing a diol via conduit 170 as a saturated liquid at atemperature which is sufficient to vaporize the water. Preferably, thediol in conduit 170 is introduced as a saturated or superheated vapor.The diol in conduit 170 is selected from the group consisting ofethylene glycol, diethylene glycol, n-butylene glycol, i-butyleneglycol, n-propylene glycol, 1,4 butanediol, cyclohexanedimethanol, andmixtures thereof. Preferably, the diol in conduit 170 is ethyleneglycol. Alternatively, an external heat source can be used to introducesufficient enthalpy to vaporize the water, which exits via conduit 165.The hydroxalky ester stream exits via conduit stream 175.

The esterification reactor zone 160 operates at a temperature that issufficient to produce a hydroxyethyl from the carboxylic acid mixture.The esterification reactor zone 160 comprises an esterification reactor.The esterification can be accomplished by any means know in the art.

In a third embodiment of this invention a process for producing aterephthalic acid/diol comprises vaporizing a decolorized terephthalicacid solution with a diol in an esterification reactor zone to remove aportion of the water to form the terephthalic acid/diol mixture; whereinthe terephthalic acid and diol subsequently reacts in the esterificationzone to form a hydroxy alkyl ester stream.

The esterification reactor zone, the decolorized terephthalic acidsolution and a process to produce the decolorized terephthalic acidsolution is described subsequently in a fourth embodiment of thisinvention.

In the forth embodiment of this invention a process for producing aterephthalic acid/diol mixture is provided as shown in FIG. #2.

Step (1) comprises mixing a crude terephthalic acid powder in conduit205 with water in conduit 215 in a mixing zone 210 to form a crudeterephthalic acid solution in conduit 220. The mixing of the crudeterephthalic powder in conduit 205 with water in conduit 215 can beaccomplished by any means known in the art. The starting feed materialis the crude terephthalic acid powder in conduit 205 with some specificphysical characteristics that differ from crude terephthalic aciddescribed in U.S. Pat. Nos. 5,095,146 and 5,175,355, herein incorporatedby reference. Specifically, the total amount of p-toluic acid and4-carboxybenzaldehyde in the crude terephthalic acid powder in conduit205 is less than about 900 ppm on a weight basis, preferably, less thanabout 500 ppm, and most preferably, less than about 250 ppm. Anothercharacteristic of the crude terephthalic powder in conduit 205 is thecolor as measured by b* is less than about 7. Preferably, the colormeasured by b* is between 4 and 6.

The mixing zone 210 can be any vessel or equipment capable of mixing thecrude terephthalic acid powder in conduit 205 with water in conduit 215.The crude terephthalic acid powder in conduit 205 is slurried in waterin conduit 215 in the mixing zone 210 to produce the crude terephthalicacid solution in conduit 220. The crude terephthalic acid and water areheated in a mixing zone 210 to a temperature of about 230° C. or higherto dissolve the crude terephthalic acid powder in conduit 205 in themixing zone 210 to produce the crude terephthalic acid solution inconduit 220. Preferably, the crude terephthalic slurry in the mixingzone 210 is heated to a temperature in the range of about 240° C. toabout 300° C. The pressure of the mixing zone is about 900 psia to about1400 psia to dissolve the crude terephthalic acid powder in conduit 205in the mixing zone 210. Generally, the concentration of crudeterephthalic acid in the crude terephthalic acid solution is about 15%to about 30% by weight, preferably, 20 to 30% by weight.

Step (2) is decolorizing the crude terephthalic acid solution in conduit220 in a reactor zone 225 to form a decolorized terephthalic acidsolution in conduit 235.

The decolorizing of the crude terephthalic acid solution in conduit 220can be accomplished by any means known in the art. Preferably, thedecolorizing can be accomplished by reacting the crude terephthalic acidsolution in conduit 220 with hydrogen in conduit 230 in the presence ofa catalyst in a reactor zone 225 to produce a decolorized terephthalicacid solution.

For the reactor zone 225, there are no special limitations in the formor construction thereof, subject to an arrangement that allows supply ofhydrogen in conduit 230 to effect intimate contact of the crudeterephthalic acid solution in conduit 220 with the catalyst in thereactor zone 225. Generally, the catalyst is usually a single Group VIIImetal or combination of Group VIII metals. Preferably, the catalyst isselected from a group consisting of palladium, ruthenium, rhodium andcombination thereof. Most preferably, the catalyst is palladium.Typically, the catalyst is supported, preferably on porous carbon.

The reactor zone 225 comprises a hydrogenation reactor which operates ata temperature of about 230° C. or higher. Preferably, the hydrogenationreactor operates in the range of about 240° C. to about 300° C. Thehydrogenation reactor operates at a pressure of about 900 psia to about1400 psia and at a hydrogen partial pressure of at least about 100 psia.Preferably, the hydrogen partial pressure is in the range of about 100to about 300 psia. By hydrogenation treatment, the characteristicallyyellow compounds in the crude terephthalic acid solution are convertedto colorless derivatives. In addition, the reactor zone converts aportion of 4-carboxybenzaldehyde to p-toluic acid. The hydrogen inconduit 230 is fed at a rate of at least about 1.5 times the molar rationecessary to convert the 4-carboxybenzaldehyde in the crude terephthalicacid solution in conduit 220 to p-toluic acid. Preferably, the hydrogen230 is fed at a rate of at least about 2.0 times the molar rationecessary to convert the 4-carboxybenzaldehyde in the crude terephthalicacid solution 220 to p-toluic acid. The b* color is between about 0.5 toabout 4 in the terephthalic acid decolorized solution in conduit 235.Preferably the b* color of the terephthalic acid solution in conduit 235is between 0.5 to 2. Most preferably the b* color in the decolorizedterephthalic acid solution in conduit 235 is between 0.5 to 1.5.

Step (3) comprises, optionally, flashing the decolorized terephthalicacid solution 235 in a flashing zone 245 to remove a portion of thewater 250 from the aqueous terepthalic acid solution 235. The flashingof the aqueous terephthalic solution 235 can be accomplished by anymeans know in the art. Typically, a vessel or a plurality of vessels areused to accomplish the flashing. In the flashing zone 245, water andresidual hydrogen can be removed as a vapor via conduit 250. The flashvessel(s) operate at a temperature of about 150° C. or higher.Preferably, the flash vessels(s) operate in the range of about 155° C.to about 260° C. The flash vessel(s) operate under a pressure of about75 psia to about 1400 psia. Specific operating ranges vary depending onthe amount of water removed via conduit 250. Alternatively, flashingzone 245 can be omitted as indicated by conduit 240.

Step (4) comprises, adding a diol in conduit 270 to the decolorizedterephthalic acid solution in conduit 255 in an esterification reactorzone 260 to remove a portion of the water via conduit 265 to form saidterephthalic acid/diol mixture in the esterification reactor zone 260.The carboxylic acid and diol react to form a hydroxyalkyester stream275. The hydroxyalkyester stream 275 comprises a hydroxyalky estercompound.

The diol in conduit 270 is introduced in such a manner as to displacethe water as the dominant slurrying liquid. This can be accomplished byintroducing a diol via conduit 270 as a saturated liquid in atemperature range of 150° C. to 300° C. Preferably, the diol in conduit270 is introduced as a saturated or superheated vapor in the temperaturerange of 150° to 300° C. in a form with sufficient enthalpy as toevaporate the water to exit via conduit 265. The diol in conduit 270 isselected from the group consisting of ethylene glycol, diethyleneglycol, n-butylene glycol, i-butylene glycol, n-propylene glycol, 1,4butanediol, cyclohexanedimethanol, and mixtures thereof. Preferably, thediol in conduit 270 is ethylene glycol. Alternatively, an external heatsource can be used to introduce sufficient enthalpy to vaporize thewater, which exits via conduit 265. The hydroxy alkyl ester streammixture exits via conduit stream 275.

The esterification reactor zone 260 operates at a temperature of about240° C. higher. Preferably the esterification reactor zone 260 operatesin the temperature range of 260° C. to 280° C. The esterificationreactor zone 260 operates under a pressure of about 40 psia to about 100psia so as to effect esterification of the terephthalic acid/diolmixture 275 to produce a hydroxyethyl ester of terephthalic acid.

In a fifth embodiment of this invention, a process for producing acarboxylic acid/diol mixture comprises removing a portion ofcontaminated water in an aqueous slurry by adding a diol in a liquorremoval zone to produce said carboxylic acid/diol mixture.

The liquor removal zone, the aqueous slurry and a process to produce theaqueous slurry are described subsequently in a sixth embodiment of thisinvention.

In the six embodiment of this invention, a process for producing acarboxylic acid/diol mixture is provided as shown in FIG. #3.

Step (1) comprises mixing a crude carboxylic acid powder in conduit 305with water in conduit 315 in a mixing zone 310 to form a crudecarboxylic acid solution in conduit 320. The mixing of the crudecarboxylic powder in conduit 305 with water in conduit 315 in the mixingzone 310 can be accomplished by any means known in the art. The startingfeed material is the crude carboxylic acid powder in conduit 305.Typically, the carboxylic acid is selected from a group consisting ofterephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, andmixtures thereof. The mixing zone 310 can be any vessel or equipmentcapable of mixing the crude carboxylic acid powder in conduit 305 withwater in conduit 315.

The crude carboxylic acid powder in conduit 305 and water in conduit 315in mixing zone 310 is heated to a temperature sufficient to dissolve thecrude carboxylic acid powder in conduit 305 in the mixing zone 310 toproduce the crude carboxylic acid solution in conduit 320. The pressureof the mixing zone 310 is a pressure sufficient to dissolve the crudecarboxylic acid powder in conduit 305 in the mixing zone 310. Generally,the concentration of crude carboxylic acid in the crude carboxylic acidsolution is about 15% to about 35% by weight.

Step (2) is decolorizing the crude carboxylic acid solution in conduit320 in a reactor zone 325 to form an decolorized carboxylic acidsolution in conduit 330.

The decolorizing of the crude carboxylic acid solution in conduit 320can be accomplished by any means known in the art. Preferably, thedecolorizing can be accomplished by reacting the crude carboxylic acidsolution in conduit 320 with hydrogen in conduit 330 in the presence ofa catalyst in a reactor zone 325 to produce a decolorized carboxylicacid solution.

For the reactor zone 325, there are no special limitations in the formor construction thereof, subject to an arrangement that allows supply ofhydrogen in conduit 330 to effect intimate contact of the crudecarboxylic. slurry 320 with the catalyst in the reactor zone. Typically,the catalyst is usually a single Group VIII metal or combination ofGroup VIII metals. Preferably, the catalyst is selected from a groupconsisting of palladium, ruthenium, rhodium and combinations thereof.Most preferably, the catalyst is palladium. Typically, the catalyst issupported, preferably on porous carbon.

The reactor zone 325 comprises a hydrogenation reactor that operates ata temperature sufficient to convert the characteristically yellowcompounds in the crude carboxylic acid solution 320 to colorlessderivatives. The b* color of in the decolorized carboxylic acid solutionin conduit 335 is between about 0.5 to about 4. Preferably the b* colorof the carboxylic acid solution in conduit 335 is between 0.5 to 2. Mostpreferably the b* color in the decolorized carboxylic acid solution inconduit 335 is between 0.5 to 1.5.

The hydrogen in conduit 330 is fed at a rate sufficient to convert thecharacteristically yellow compounds in the crude carboxylic slurry inconduit 320 to colorless derivatives; wherein the b* color is betweenabout 0.5 to about 4 in the decolorized carboxylic acid solution inconduit 335.

Step (3) comprises crystallizing the decolorized carboxylic acidsolution in conduit 335 in a crystallization zone 345 to form an aqueousslurry in conduit 355.

The crystallization zone 345 comprises a vessel or plurality of vesselscapable of removing water from the decolorized carboxylic acid solutionin conduit 335 to produce an aqueous slurry in conduit 355. Typically,the vessels comprise at least one crystallizer. Examples of such systemscan be found in U.S. Pat. Nos. 5,567,842 and 3,931,305, hereinincorporated by reference. Generally, the aqueous slurry in conduit 355has a carboxylic acid concentration of from 10 to 60 weight percent. Thetemperature range of the carboxylic acid solution in the crystallizationzone 345 is that which is sufficient to remove a portion of the water.

Step (4) comprises removing a portion of contaminated water via conduit365 in the aqueous slurry 355 by adding a diol in conduit 370 in aliquor removal zone 360 to produce the carboxylic acid/diol mixture inconduit 375.

The purpose of the liquor removal zone 360 is to replace thecontaminated-water with a diol in conduit 370. The contaminated watercomprises water and typical contaminants. The diol in conduit 370 isselected from the group consisting of ethylene glycol, diethyleneglycol, n-butylene glycol, i-butylene glycol, n-propylene glycol, 1,4butanediol, cyclohexanedimethanol, and mixtures thereof. Preferably, thediol in conduit 370 is ethylene glycol. The diol in conduit 370 isintroduced into the liquor removal zone 360 via conduit 370. The removalof the contaminated water via conduit 365 in the liquor removal zone 360can be accomplished using variety of techniques including, but notlimited to, cyclones, centrifuges, and filters. The key factor in theliquor removal zone 360 is to select a temperature range where thetypical contaminants preferably remain with the aqueous mother liquorinstead of remaining with the carboxylic acid. The resultant carboxylicacid/diol mixture is removed via conduit 375. The resultant carboxylicacid/diol mixture in conduit 375 is adequate as feed material for theesterification of carboxylic acid with the diol to produce the ester ofcarboxylic acid.

In a seventh embodiment of this invention, a process for producing aterephthalic acid/diol mixture comprises removing a portion of thep-toluic contaminated water in a terephthalic acid aqueous aqueousslurry by adding a diol in a liquor removal zone to produce saidterephthalic acid/diol mixture.

The liquor removal zone, the terephthalic acid aqueous slurry and aprocess to produce the aqueous slurry are described subsequently in aneight embodiment of this invention.

In the eight embodiment of this invention, a process for producing aterephthalic acid/diol mixture is provided as shown in FIG. #4.

Step (1) comprises mixing a crude terephthalic acid powder in conduit405 with water in conduit 415 in a mixing zone 410 to form a crudeterephthalic acid solution in conduit 420. The mixing of the crudeterephthalic powder in conduit 405 with water in conduit 415 in themixing zone 410 can be accomplished by any means known in the art. Thestarting feed material is the crude terephthalic acid powder in conduit405. The total amount of p-toluic acid and 4-carboxybenzaldehyde in thecrude terephthalic acid powder in conduit 405 is less than about 6000ppm on a weight basis. Another characteristic of the crude terephthalicpowder 405 is the color as measured by b* is less than about 7.Preferably the color measured by b* is between 4 and 6. This crudeterephthalic acid powder in conduit 405 is introduced into a mixing zone410. The mixing zone 410 can be any vessel or equipment capable ofmixing the crude terephthalic acid powder in conduit 405 with water inconduit 415.

The crude terephthalic acid powder and water are heated to a temperatureof about 230° C. or higher to dissolve the crude terephthalic acidpowder in conduit 405 in the mixing zone 410 to produce the crudeterephthalic acid solution in conduit 420. Preferably, the crudeterephthalic acid solution in the mixing zone 410 is heated to atemperature in the range of about 240° C. to about 300° C. The pressureof the mixing zone 410 is about 900 psia to about 1400 psia to dissolvethe crude terephthalic acid powder in conduit 405 in the mixing zone410. Generally, the concentration of crude terephthalic acid powder 405in the crude terephthalic acid solution 420 is in a range of about 15%to about 35% by weight, preferably 20 to 30% by weight.

Step (2) is decolorizing the crude terephthalic acid solution in conduit420 in a reactor zone 425 to form a decolorized terephthalic acidsolution in conduit 435.

The decolorizing of the crude carboxylic acid solution in conduit 420can be accomplished by any means known in the art. Preferably, thedecolorizing can be accomplished by reacting the crude carboxylic acidsolution in conduit 420 with hydrogen in conduit 430 in the presence ofa catalyst in a reactor zone 425 to produce a decolorized carboxylicacid solution.

For the reactor zone 425, there are no special limitations in the formor construction thereof, subject to an arrangement that allows supply ofhydrogen in conduit 430 to effect contact of the crude terephthalicslurry 420 with the catalyst in the reactor zone. The catalyst isusually a single Group VIII metal or combination of Group VIII metals.Preferably, the catalyst is selected from a group consisting ofpalladium, ruthenium, rhodium and combinations thereof. Most preferably,the catalyst is palladium. Typically, the catalyst is supported,preferably on porous carbon.

The reactor zone 425 comprises a hydrogenation reactor which operates ata temperature of about 230° C. or higher. Preferably the hydrogenationreactor operates in the range of about 240° C. to about 300° C. Thehydrogenation reactor operates at a pressure of about 900 psia to about1400 psia and at a hydrogen partial pressure of at least about 100 psia.Preferably, the hydrogen partial pressure is in the range of about 100to about 300 psia. By hydrogenation treatment, the characteristicallyyellow compounds in the crude terephthalic acid solution 420 areconverted to colorless derivatives. In addition, the reactor zoneconverts a portion of 4-carboxybenzaldehyde to p-toluic acid.

The hydrogen in conduit 430 is fed at a rate of at least about 1.5 timesthe molar ratio necessary to convert the 4-carboxybenzaldehyde in thecrude terephthalic slurry 420 to p-toluic acid. Preferably the hydrogen430 is fed at a rate of at least about 2.0 times the molar rationecessary to convert the 4-carboxybenzaldehyde in the crude terephthalicslurry 420 to p-toluic acid. The b* color is between about 0.5 to about4 in the decolorized terephthalic acid solution in conduit 435.Preferably the b* color of the terephthalic acid solution in conduit 435is between 0.5 to 2. Most preferably the b* color in the decolorizedterephthalic acid solution in conduit 435 is between 0.5 to 1.5.

Step (3) comprises crystallizing said decolorized terephthalic acidsolution in conduit 435 in a crystallization zone 445 to form aterephthalic acid aqueous slurry in conduit 455.

The crystallization zone 445 comprises a vessel or plurality of vesselscapable of removing water via conduit 450 from the decolorizedterephthalic acid solution in conduit 435 to produce an terephthalicacid aqueous slurry in conduit 455. Typically the vessels comprise atleast one crystallizer as previously described. Generally, theterephthalic acid aqueous slurry in conduit 455 has a terephthalic acidconcentration of from 10 to 60 weight percent, preferably from 20 to 40weight percent. Examples of such systems can be found in U.S. Pat. Nos.5,567,842 and 3,931,305 both of which are herein incorporated byreference. The temperature range of the terephthalic acid aqueous slurryin conduit 455 is from about 120° C. to about 270° C. The pressure rangeof the crystallizing is from about 75 to about 1400 psia.

Step (4) comprises removing a portion of p-toluic acid contaminatedwater via conduit 465 in the terephthalic acid aqueous slurry 455 byadding a diol in conduit 470 in a liquor removal zone 460 to producesaid terephthalic acid/diol mixture in conduit 475.

The purpose of the liquor removal zone 460 is to replace the p-toluicacid contaminated water with a diol in conduit 470. The diol in conduit470 is selected from a group consisting of ethylene glycol, diethyleneglycol, n-butylene glycol, i-butylene glycol, n-propylene glycol, 1,4butanediol, cyclohexanedimethanol, and mixtures thereof. Preferably, thediol in conduit 470 is ethylene glycol. The diol in conduit 470 isintroduced into the liquor removal zone 460 via conduit 470. The removalof the p-toluic acid contaminated water via conduit 465 in the liquorremoval zone 460 can be accomplished using variety of techniquesincluding, but not limited to, cyclones, centrifugation, and filtration.The key factor in the liquor removal zone 460 is to select a temperaturerange where the p-toluic acid and 4-carboxybenzaldehyde will preferablyremain with the aqueous mother liquor instead of remaining with theterephthalic acid. The liquor removal zone 460 operates in a range ofabout 120° C. to about 270° C., preferably in the range of 120° C. to150° C. The p-toluic acid contaminated water is removed via conduit 465.The resultant terephthalic acid/diol mixture is removed via conduit 475.The resultant terephthalic acid/diol mixture in conduit 475 is adequateas feed material for the esterification of terephthalic acid with a diolto produce the ester of terephthalic acid.

1. A process for producing a carboxylic acid/diol mixture, said processcomprising adding a decolorized carboxylic acid solution comprising acarboxylic acid and water in an esterification reactor zone and adding adiol to said decolorized carboxylic acid solution in said esterificationzone; wherein said diol is added to remove at least 5% by weight of saidwater vaporization from said decolorized carboxylic acid solution toform said carboxylic acid/diol mixture.
 2. A process according to claim1 wherein said carboxylic acid is selected from a group consisting ofterephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, andmixtures thereof.
 3. A process for producing a carboxylic acid/diolmixture, said process comprising the following steps: (a) mixing a crudecarboxylic acid powder with water in a mixing zone to form a crudecarboxylic acid solution; wherein said carboxylic acid is selected froma group consisting of terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, and mixtures thereof; (b) decolorizing said crudecarboxylic acid solution in a reactor zone to produce a decolorizedcarboxylic acid solution; (c) optionally, flashing said decolorizedcarboxylic acid solution in a flashing zone to remove a portion of thewater from said decolorized carboxylic acid solution; and (d) addingsaid decolorized carboxylic acid solution comprising a carboxylic acidand water in an esterification reactor zone and adding a diol to saiddecolorized carboxylic acid solution in said esterification zone;wherein said diol is added to remove at least 5% by weight of the saidwater by vaporization from said decolorized carboxylic acid solution toform said carboxylic acid/diol mixture.
 4. A process according to claim3 wherein said decolorizing is accomplished by reacting said crudecarboxylic acid solution with hydrogen in the presence of a catalyst ina reactor zone to produce said decolorized carboxylic acid solution;wherein said catalyst comprises a group VIII metal or combinationthereof.
 5. A process according to claim 1 or 3 wherein said diol isselected from the group consisting of ethylene glycol, diethyleneglycol, n-butylene glycol, i-butylene glycol, n-propylene glycol, 1,4butanediol, cyclohexanedimethanol, and mixtures thereof.
 6. A processfor producing a terephthalic acid/diol mixture, said process comprisingthe following steps: (a) mixing a crude terephthalic acid powder withwater in a mixing zone to form a crude terephthalic acid solution; (b)decolorizing said crude terephthalic acid solution in a reactor zone toform a decolorized terephthalic acid solution; (c) optionally, flashingsaid decolorized terephthalic acid solution in a flashing zone to removea portion of the water from said decolorized terephthalic acid solution;and (d) adding a diol to said decolorized terephthalic acid solutioncomprising terephthalic acid and water in an esterification reactorzone; wherein said diol is added to remove at least 5% of said water byvaporization from said decolorized terephthalic acid solution to formsaid terephthalic acid/diol mixture; and wherein said terephthalic acidand diol subsequently reacts in said esterification zone to form ahydroxy alkyl ester stream.
 7. A process according to claim 6 whereinsaid decolorizing is accomplished by reacting said crude terephthalicacid solution with hydrogen in the presence of a catalyst in saidreactor zone to produce said decolorized terephthalic acid solution;wherein said catalyst comprises a group VIII metal or combinationthereof.
 8. A process according to claim 6 wherein said crudeterephthalic acid powder contains a concentration of p-toluic acid and4-carboxybenzaldehyde less than about 900 ppm by weight.
 9. A processaccording to claim 6 wherein said diol is selected from the groupconsisting of ethylene glycol, diethylene glycol, n-butylene glycol,i-butylene glycol, n-propylene glycol, 1,4 butanediol,cyclohexanedimethanol, and mixtures thereof.
 10. A process according toclaim 6 wherein said diol is ethylene glycol.
 11. A process according toclaim 8 wherein said crude terephthalic powder has a b* color less thanabout
 7. 12. A process according to claim 6 wherein said mixing occursat a pressure between about 900 psia to about 1400 psia.
 13. A processaccording to claim 6 wherein the concentration of crude terephthalicacid in said crude terephthalic acid solution is in a range betweenabout 15% to about 35% by weight.
 14. A process according to claim 7wherein said reacting occurs at a temperature greater than about 230° C.15. A process according to claim 7 wherein said reacting occurs at apressure between about 900 psia to about 1400 psia.
 16. A processaccording to claim 7 wherein said reacting occurs at a hydrogen flowrate of about 1.5 times the molar ratio necessary to convert4-carboxybenzaldehyde to p-toluic acid in said crude terephthalic acidsolution.
 17. A process according to claim 6 wherein said flashingoccurs at a temperature greater than about 150° C.
 18. A processaccording to claim 6 wherein said flashing occurs at a pressure betweenabout 75 psia to about 1400 psia.
 19. A process according to claim 6wherein said adding occurs at a temperature greater than about 240° C.20. A process according to claim 6 wherein said diol is introduced intosaid esterification reactor zone at a temperature between about 150° C.to about 300° C.
 21. A process according to claim 6 wherein said addingoccurs at a pressure between about 40 psia to about 100 psia.
 22. Aprocess for producing a terephthalic acid/ethylene glycol mixture, saidprocess comprising the following steps: (a) mixing a crude terephthalicacid powder with water in a mixing zone to form a crude terephthalicacid solution wherein said mixing zone is operated at a temperaturegreater than about 230° C., wherein the pressure in said mixing zone isin a range of about 900 psia to about 1400 psia; wherein theconcentration of said crude terephthalic acid in said crude terephthalicacid solution is in a range of about 15% to about 35% by weight; whereinthe color of said crude terephthalic acid powder is less than about 7measured by b*; wherein the total concentration of p-toluic and4-carboxybenzaldehyde in said crude terephthalic acid powder is lessthan about 900 ppm by weight; (b) decolorizing said crude terephthalicacid solution with hydrogen in a reactor zone in the presence of acatalyst to form a decolorized terephthalic acid solution; wherein saidcatalyst comprises a group VIII metal or combination thereof; whereinsaid hydrogen is introduced into said reactor zone at a flow rate ofabout 1.5 times the molar ratio necessary to convert4-carboxybenzaldehyde to p-toluic acid in said crude terephthalic acidsolution; wherein said reacting occurs at a temperature greater thanabout 230° C., wherein said reacting occurs at a pressure between about900 psia to about 1400 psia; (c) optionally, flashing said decolorizedterephthalic acid solution in a flashing zone to remove a portion ofwater from said decolorized terephthalic acid solution wherein saidflashing zone comprises at least one flash vessel operated at a pressurein the range of about 75 psia to about 1400 psia; and wherein saidvessel is operated at a temperature in a range greater than about 150°C.; and (d) adding a diol to said decolorized terephthalic acid solutioncomprising terephthalic acid and water in an esterification reactorzone; wherein said diol is added to remove at least 5% by weight ofwater by vaporization to form said terephthalic acid/ethylene glycolmixture; and wherein said carboxylic acid and diol subsequently reactsto form a hydroxy alkyl ester stream; wherein said diol is ethyleneglycol at a temperature in a range of about 150° C. to about 300° C.